1. Field of the Invention
The present invention relates to a drive force transmitting mechanism including two gears that rotate while being engaged with each other, in which one of the gears that is movably supported is configured to contact and separate from the other gear. The present invention also relates to an image forming apparatus including the drive force transmitting mechanism.
2. Discussion of the Related Art
A drive force transmitting mechanism that rotates a drive gear and a driven gear while being engaged with each other to transmit a drive force from the drive gear to the driven gear has been widely used. For example, in an image forming apparatus such as a copying machine, a facsimile machine, a laser beam printer, or other similar image forming apparatus, in which a drive motor acting as a drive source is fixed to a case of the image forming apparatus, a driven unit including a photoreceptor and a developing device, for example, is slidably supported in the case such that the driven unit is detachably attached to the case. In this image forming apparatus, a driven gear provided in the driven unit contacts and separates from a drive gear provided to the case of the image forming apparatus by a sliding movement of the driven unit. In another image forming apparatus described in Published Japanese patent application No. 11-119583, a cover mounted with driven elements such as pairs of sheet conveyor rollers is configured to be opened and closed relative to a case of the image forming apparatus to which a motor is fixed. In this image forming apparatus, a driven gear provided to the cover contacts and separates from a drive gear provided to the case of the image forming apparatus by opening and closing the cover.
In these image forming apparatuses, the following forces are exerted on the movable driven gear at a position where the drive gear and the driven gear rotate while being engaged with other, in reaction to the rotations of the drive gear and the driven gear. For example, a force for further intruding the driven gear into the drive gear and a force for moving the driven gear away from the drive gear are exerted on the driven gear. If the driven gear excessively intrudes into the drive gear, the drive gear and the driven gear are locked. If the driven gear moves away from the drive gear, a distance between an axial center of the drive gear and an axial center of the driven gear increases, thereby making the rotational speed of the driven gear unstable.
To prevent an excessive intrusion of a driven gear into a drive gear, a drive force transmitting mechanism including a disk portion has been used. Specifically, the disk portion is provided to at least one of the drive gear and the driven gear such that the disk portion and a gear portion of the at least one of the drive gear and the driven gear are disposed side by side in the rotational axial direction of the at least one of the drive gear and the driven gear. The disk portion has a diameter greater than that of the gear portion. In this drive force transmitting mechanism, an excessive approach of one of the gears (first gear) to the other gear (second gear) is prevented by abutting the disk portion provided to the first gear against a shaft member that supports the second gear. However, in this drive force transmitting mechanism, if the disk portion provided to the first gear is abutted against the shaft member that supports the second gear, the rotation of the first gear may be hampered, thereby increasing a drive torque of a drive motor acting as a drive source.
Further, to prevent a movement of a driven gear away from a drive gear, a drive force transmitting mechanism, in which a movable support element that supports the driven gear and its shaft member is biased toward the drive gear with a spring, has been used. In this drive force transmitting mechanism, the movement of the driven gear away from the drive gear is hampered by the biasing force of the spring. By doing so, the driven gear may stably rotate in the vicinity of the drive gear. However, in this drive force transmitting mechanism, the movement of the driven gear away from the drive gear may not be securely prevented for the following reasons.
Generally, in a configuration in which the driven gear is configured to be movable with the support element that supports the driven gear, a loose movement of the support element is allowed to slide or open/close the support element smoothly. With the loose movement of the support element, the support element can move within a predetermined range in a direction (hereafter referred to as “a loosing direction”) different from a sliding direction or an opening/closing direction of the support element. In this loose movement of the support element, the support element that supports the driven gear which rotates while being engaged with the drive gear, typically moves away from the drive gear in the loosing direction as well as in the sliding direction or the opening/closing direction. Thus, even if the movement of the support element in the sliding direction or the opening/closing direction can be prevented with the biasing force of the spring, the movement of the support element in the loosing direction may not be hampered. Even if the movement of the support element in the loosing direction can be prevented with biasing forces of a plurality of springs, the movement of the support element may not be adequately prevented if the biasing forces of the springs get weakened due to the deterioration of the springs.
The above-described problems may similarly occur when a drive roller moves relative to a driven roller, that is, the drive roller contacts and separates from the driven roller.
Therefore, it is desirable to provide a drive force transmitting mechanism that prevents an excessive intrusion of a movable second gear into a first gear while controlling the increase of a drive torque of a drive source, and that prevents the movement of the second gear away from the first gear engaged with the second gear.